Starting/generating system with multi-functional circuit breaker

ABSTRACT

A pre-charge circuit limits in-rush currents on a direct current (DC) link that includes a first DC link bus and a second DC link bus. The pre-charge circuit includes a switching device connected in series with the first DC link bus. The switching device has an ON state in which power flow is enabled on the DC link and an OFF state in which power is disabled on the DC link. A controller selectively modulates the state of the switching device to limit in-rush currents on the DC link.

BACKGROUND

The present invention is related generally to electrical power systems,and in particular to starting/generating systems.

Starting/generating systems refer to systems capable of operating ineither a starting mode in which the system operates as a motor toaccelerate a rotor portion to a desired speed or in a generating mode inwhich the system operates as a generator to convert mechanical energyprovided by the rotor portion into electrical energy for distribution toattached loads.

Depending on the mode, various electronic circuits are required toprovide the desired functionality. During starting, a pre-charging orsoft-start circuit may be employed to prevent large in-rush currentsfrom damaging a DC link capacitor(s). For example, prior art pre-chargecircuits may employ a switching device and resistor (connected inparallel with one another) connected in series on the DC link busbetween the power supply and the inverter/rectifier circuit. Theswitching device is turned OFF in order to force current through theresistor connected in parallel with the switching device, therebylimiting the current provided to the inverter/rectifier circuit and DClink capacitor. However, this topology does not provide functionalitybeyond pre-charge operations. Alternatively, the switching device andresistor can be placed in series with the DC link capacitor, which isconnected between the DC link buses in parallel with theinverter/rectifier circuit. In this way, the switching device is notrequired to be capable of carrying the full inverter/rectifier current,but the presence of the switching device in series with the capacitordecreases the performance of the DC link capacitor, due to theresistance of the switching device.

In addition to circuits or components employed to provide pre-chargingfunctionality, starting/generating systems employ additionalhardware/circuits to implement functions such as battery charging, powerflow enablement, and fault protection. These additional hardware/circuitcomponents add to the overall cost and weight of starting/generatingsystems, reduction of which is desirable.

SUMMARY

A pre-charge circuit limits in-rush currents on a direct current (DC)link that includes a first DC link bus and a second DC link bus. Thepre-charge circuit includes a switching device connected in series withthe first DC link bus. The switching device has an ON state in whichpower flow is enabled on the DC link and an OFF state in which power isdisabled on the DC link. A controller selectively modulates the state ofthe switching device to limit in-rush currents during pre-charge on theDC link. In addition, the pre-charge circuit can be used subsequent topre-charge of the DC link to implement additional functionality withrespect to power flow on the DC link.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a circuit diagram illustrating a starting/generating systemaccording to an embodiment of the present invention.

DETAILED DESCRIPTION

FIG. 1 is a circuit diagram of starting/generating system 10 accordingto an embodiment of the present invention. System 10 includes rotorportion 12 and stator portion 14. Rotor portion 12 includes motive powershaft 16, main generator portion field winding 18, rotating rectifier20, three-phase armature windings 22, permanent magnet generator (PMG)magnets 24, and prime mover 26. Stator portion 14 includes filtercircuit 28 (which includes capacitor C1, inductor L1, and diode D1),solid state circuit breaker 30, controller 32, DC link buses 34 a and 34b (collectively DC link 34), DC link capacitor C2, inverter/rectifier36, main generator portion armature winding 38, exciter field winding40, H-bridge 42, three-phase (PMG) stator windings 44, rectifier 46 andcontrol electronics 48. Power source 50 represents a generic powersource for providing DC power to starting/generating system 10 duringstarting operations and DC load 52 represents a generic load thatreceives power from starting/generating system 10 during generatingoperations. The embodiment shown in FIG. 1 represents the systememployed with respect to a wound field synchronous generator, but wouldbe applicable to other generator systems such as flux regulatedpermanent magnet generators and other well-known starting/generatingsystems.

In the starting mode, electrical energy provided by DC power source 50is converted to alternating current (AC) power by inverter/rectifier 36(operating as an inverter). Further, the exciter power converterH-bridge 42 delivers AC power to the exciter field winding 40. Theexciter acts as a rotary transformer having a primary winding comprisingthe field winding 40 and secondary windings comprising the armaturewindings 22 so that AC power is induced in the armature windings 22. TheAC power is rectified by the rotating rectifier 20 and applied as DCpower to the main generator portion field winding 18. The AC power isprovided to main generator portion armature winding 38, which interactswith main generator portion field winding 18 to generate motive forcethat causes rotor portion to rotate.

In the generating mode, mechanical energy provided by prime mover 26 isconverted to electrical energy. In particular, rotation of PMG magnets24 generates electrical energy in three-phase PMG stator windings 44.Rectifier 46 converts the AC voltage to a DC voltage that is selectivelysupplied to exciter field winding 40 via H-bridge 42. The DC excitationprovided by exciter field winding 40 interacts with three-phase armaturewindings 22. The DC current in exciter field winding 40 is controlled inresponse to the output DC voltage applied to DC load 52 by a voltageregulator located within control electronics 48. The AC voltagegenerated by three-phase armature windings 22 is converted to DC byrotating rectifier 20 and supplied to main generator portion fieldwinding 18. The rotating field generated by field winding 18 interactswith main generator portion armature winding 38 to generate AC voltage.Inverter/rectifier 36 (operating as a rectifier) converts the AC voltageto DC voltage that is supplied to DC load 52. In addition, the DCvoltage may be used to charge an attached battery (for example, DC powersource 50). The dual functionality of starting/generating system 10 isillustrated visually by switch S1, which indicates thatstarting/generating system may receive power from DC source 50 (startingmode) and may supply power to a DC load 52 (generating mode). Althoughin some embodiments, the DC power source (i.e., battery) may also act asa DC load during re-charging of the battery from power generated bystarting/generating system 10.

Solid-state circuit breaker 30 is connected on DC link bus 34 a inseries between inverter/rectifier 36 and DC power source 50 (or DC load52, depending on the mode of operation). Solid-state circuit breaker 30combines functionality previously provided by a plurality of individualcircuits. During pre-charge (i.e., soft-starting) of DC link capacitorC2, the state of solid-state circuit breaker 30 is selectively modulated(i.e., turned ON and OFF) to control in-rush currents. During a startingmode (subsequent to pre-charge), solid-state circuit breaker 30 isselectively controlled to enable power flow from DC power source 50 toinverter/rectifier 36 and to disable power flow from inverter/rectifier36 to DC power source 50. During a generating mode, solid-state circuitbreaker 30 enables power provided by the generator to be supplied to DCload 52, and is selectively controlled (i.e., turned OFF) in response tofault conditions such as short-circuit conditions, overload conditions,etc., to prevent damage to the generator and/or DC load 52. Also duringthe generating modes, solid-state circuit breaker 30 is selectivelymodulated to provide a desired current profile for battery chargingoperations.

Pre-charging (i.e., soft-starting) of DC link capacitor C2 preventslarge currents from damaging DC link capacitor C2 during an initialapplication of power from DC power source 50. Pre-charging functionalityis provided by selectively modulating solid-state circuit breaker 30(i.e., turning it ON and OFF). In-rush current is a function of thevoltage applied to the capacitor and the characteristics of thecapacitor. By selectively modulating solid-state circuit breaker 30, thevoltage applied to DC link capacitor C2 can be controlled, therebylimiting the in-rush current provided to DC link capacitor C2.

In one embodiment, controller 32 monitors one or more parameters andbased on the monitored parameters selectively controls the modulation ofsolid-state circuit breaker 30. The operation of controller 32 may beclosed-loop or open-loop, depending on the application. In an open-loopapplication, the duty cycle of solid-state circuit breaker 30 isselectively controlled without feedback regarding the voltage or currentprovided to DC link capacitor C2. For instance, controller 32 maycontrol the duty cycle based on the length of time from application ofpower from DC power source 50, with the duty cycle increasing based onsome function (linearly or non-linearly) until the expiration of thepre-charge cycle. At the end of the pre-charge cycle solid-state circuitbreaker 30 is turned ON (i.e., maintained in the ON state continuously)such that DC power source 50 supplies power to inverter/rectifier 36 forstarting operations.

In closed-loop applications, controller 32 monitors one or moreparameters and in response selectively controls the modulation (i.e.,duty cycle) of solid-state circuit breaker 30. Examples of parametersused to determine the modulation of solid-state circuit breaker 30include the monitored DC link voltage, the monitored DC link capacitorcurrent, and/or the monitored DC link current. Based on these parameterscontroller 32 can selectively control in-rush currents during pre-chargeof DC link capacitor C2. For example, because the in-rush current isdependent on the voltage supplied to DC link capacitor C2, the monitoredDC link voltage may be used as feedback to selectively control thein-rush current. Controller 32 monitors the voltage across DC link andin response selectively modulates solid-state circuit breaker 30 toprovide the desired pre-charge of capacitor C2. As the voltage across DClink increases, the duty cycle of solid-state circuit breaker 30 isselectively increased until some pre-charge threshold, at which timesolid-state circuit breaker is maintained in the ON state (continuously)to provide starting power to inverter/rectifier 36. In otherembodiments, the monitored DC link current and/or DC link capacitorcurrent can be used as feedback to selectively control the in-rushcurrent. Once again, the duty cycle is increased until at the end of thepre-charge cycle solid-state transistor 30 is maintained in the ON state(continuously) to provide starting power to inverter/rectifier 36.Monitoring the in-rush current directly provides feedback regarding theoutput to be controlled, but requires additional hardware (e.g., currentsensors) to implement.

Solid-state circuit breaker 30 may also be used to selectively enablepower flow during starting operations and may be used to disable powerflow in response to the voltage generated by the generator exceeding thevoltage provided by DC power source 50 (prior to supplying voltage fromthe generator to DC load 52). For example, having pre-charged DC linkcapacitor C2, solid-state circuit breaker 30 is selectively controlled(i.e., turned ON) to enable power flow from DC power source 50 toinverter/rectifier 36 to operate in a starting mode that may includeaccelerating rotor portion 12, igniting a combustor (for gas turbineengines) and assisting in accelerating the rotor portion 12 to a desiredspeed following successful ignition. Subsequent to these stages,solid-state circuit breaker 30 is turned OFF to prevent power generatedby starting/generating system 10 from flowing into DC power source 50.For example, when the voltage provided by starting/generating system 10(i.e., the DC power provided by inverter/rectifier 36) exceeds themagnitude of the DC voltage provided by DC power source 50, thencontroller 32 turns solid-state circuit breaker 30 OFF to disable powerflow from DC power source to starting/generating system 10 (or viceversa).

Circuit breaker 30 may also be employed during the generating mode toprovide the desired current profile for optimal battery charging. Forexample, DC power source 50 may be a battery that requires re-chargingafter each starting operation. Rather than employ a separate circuit formonitoring and controlling the current profile provided to the battery(i.e., DC power source 50), controller 32 monitors the current providedto the battery and selectively modulates solid-state circuit breaker 30to provide the desired current profile for charging. Typically, thecurrent provided to the battery is sensed and provided as feedback tocontroller 32, although in other embodiments other parameters may bemonitored and used in feedback to control the current profile duringbattery charging operations.

Solid-state circuit breaker may also be employed to provide faultprotection during the generator mode by selectively disabling power flowfrom starting/generating system 10 to DC load 30 in response to adetected fault condition. For example, controller 32 may monitor one ormore parameters, such as DC link voltage, DC link current and/or DC linkcapacitor current to detect faults such as short-circuits. In responseto a detected fault, controller 32 causes solid-state circuit breaker 30to turn OFF to prevent excessive currents from being provided to DC load52. In one embodiment, the fault protection provided by solid-statecircuit breaker is not activated until a detected fault has existed onthe link for a predetermined period of time, to prevent transientconditions from initiating fault protection. Additional parameterswell-known in the art for detecting fault conditions may also bemonitored by controller 32. In addition, other controllers, such ascontrol electronics 48, may provide input to controller 32 regardingdetected fault conditions. In response to these inputs, controller 32selectively activates fault protection by turning OFF solid-statecircuit breaker 30.

The present invention provides a starting/generating circuit topology inwhich a solid-state circuit breaker is employed to implement a number offunctions required at various stages starting/generating systemoperation. The solid-state circuit breaker is connected in series on aDC link bus and is selectively controlled (e.g., turned ON and OFF) toprovide the desired functionality.

While the invention has been described with reference to an exemplaryembodiment(s), it will be understood by those skilled in the art thatvarious changes may be made and equivalents may be substituted forelements thereof without departing from the scope of the invention. Inaddition, many modifications may be made to adapt a particular situationor material to the teachings of the invention without departing from theessential scope thereof. Therefore, it is intended that the inventionnot be limited to the particular embodiment(s) disclosed, but that theinvention will include all embodiments falling within the scope of theappended claims.

1. A pre-charge circuit for limiting in-rush current on a direct current(DC) link having a first DC link bus and a second DC link bus, thepre-charge circuit comprising: a switching device connectable in serieswith the first DC link bus, wherein the switching device operates ineither an ON state or an OFF state; and a controller connected toselectively modulate the state of the switching device during apre-charge cycle to limit in-rush currents on the DC link.
 2. Thepre-charge circuit of claim 1, wherein the switching device is asolid-state circuit breaker.
 3. The pre-charge circuit of claim 1,wherein the controller employs open-loop control of the switching devicemodulation.
 4. The pre-charge circuit of claim 1, wherein the controllermonitors one or more parameters selected from the group consisting of:DC link voltage, DC link current, DC link capacitor current, andcombinations thereof.
 5. The pre-charge circuit of claim 4, wherein thecontroller employs closed-loop control of the switching devicemodulation based on the one or more monitored parameters.
 6. Thepre-charge circuit of claim 1, wherein the controller selectivelyenables power flow on the DC link by turning the switching device ON anddisables power flow on the DC link by turning the switching device OFF.7. The pre-charge circuit of claim 1, wherein the controller monitorscurrent provided by the DC link to a battery and selectively modulatesthe state of the switching device to create a desired current profilefor charging the battery.
 8. The pre-charge circuit of claim 1, whereinthe controller detects fault conditions and selectively turns theswitching device OFF to disable power flow on the DC link.
 9. A methodfor limiting in-rush currents on a direct current (DC) link having afirst DC link bus and a second DC link bus, the method comprising:modulating a state of a switching device connected in series on thefirst DC link bus to limit in-rush currents during pre-charge of acapacitor connected between the first DC link bus and the second DC linkbus.
 10. The method of claim 9, further including: monitoring one ormore parameters selected from the group consisting of: DC link voltage,DC link current, DC link capacitor current, and combinations thereof;and controlling the modulation of the switching device based on the oneor more monitored parameters.
 11. The method of claim 9, furtherincluding: turning the switching device ON to enable power flow on theDC link; and turning the switching device OFF to disable power flow onthe DC link.
 12. The method of claim 9, further including: monitoringcurrent provided by the first DC link bus to a battery; and modulatingthe state of the switching device to create a desired current profilefor charging the battery.
 13. The method of claim 9, further including:monitoring one or more parameters selected from the group consisting of:DC link voltage, DC link current, DC link capacitor current, andcombinations thereof; detecting fault conditions based on the one ormore monitored parameters; and selectively turning the switching deviceOFF to disable power flow on the DC link in response to a detected faultcondition.